1. Field of the Invention
The present invention relates to an antenna to be adapted to an antenna which is used in, for example, wireless LAN.
2. Description of the Related Art
A dipole antenna is non-directional in a horizontal plane. While non-directivity has an advantage of ensuring radiation in all the directions on a horizontal plane, it raises a problem of making it difficult to set an electric scatterer nearby. If an electric scatterer (a metal body, any other dielectric substance or the like) overlaps the peripheral portion of the antenna where the radiation level is high, electromagnetic coupling causes a current which should originally flow to the antenna to flow toward the electric scatterer. This results in deteriorations of the antenna characteristics, such as shifting of the resonance frequency of the antenna and reduction in the radiation efficiency of the antenna.
Recently, there is a need for a built-in dipole antenna from viewpoints of making devices compact and the design. The incorporation of an antenna allows a metal casing, a metal heat sink, a printed wiring board or the like to be positioned close to the antenna, leading to the aforementioned deteriorations of the antenna characteristics.
Possible solutions to this problem include increasing the distance between the antenna and the radio scatterer, and insertion of a radio absorber between the antenna and the radio scatterer. The method of increasing the distance hinders miniaturization of the whole antenna, and the insertion of the radio absorber stands in the way of reducing the cost. To incorporate a dipole antenna in a smaller space in a radio device at a lower cost, it is desirable to control the directivity of the antenna to spatially avoid the nearby radio scatterer.
As one way of controlling the directivity of an antenna, controlling the antenna directivity by using a linear parasitic element has been proposed (see JP-A-2001-185947(Patent Document 1)). The antenna described in Patent Document 1 has a common dipole 5 having a full length of λ/2 (λ: wavelength corresponding to the transmission frequency), shown in FIG. 1A. Further, linear parasitic elements 61 to 6n each having a length of λ/2 are disposed at positions separate from the axis of the common dipole 5 by a distance D2 so as to enclose the common dipole 5. The linear parasitic elements 61 to 6n each have a cross section of a radius D1.
A U-shaped parasitic element 7 is disposed in close vicinity of one end of the common dipole 5. The U-shaped parasitic element 7 includes a bottom portion 7a formed of a cylindrical conductor having a radius D3 and a length L3, and two arm portions 7b each formed of a cylindrical conductor having the radius D3 and a length L2. The U-shaped parasitic element 7 serves to match the impedance of the common dipole 5 with the impedance of the linear parasitic elements.
The electromagnetic wave from the common dipole 5 induces a resonance current in the linear parasitic elements 61 to 6n, so that the electromagnetic waves radiated from the linear parasitic elements 61 to 6n are combined with the electromagnetic wave radiated from the common dipole 5 to change the radiation directivity.